Periodically poled crystals of non-linear optical, electro-optical and acousto-optical crystals, such as potassium titanyl phosphate (KTiOPO4) designated as KTP and other crystals within the same crystal family, such as the isomorphs KTA known as potassium titanyl arsenide, RTA known as rubidium titanyl arsenide, CTA known as cesium titanyl arsenide, RTP known as rubidium titanyl phosphate, and others are preferably grown by the so-called flux method.
Periodically poled crystals have been used mainly for non-linear optical frequency mixing, and in particular to generate light at new wavelengths on the basis of light at available laser wavelengths. There is at present a desire for novel radiation sources based on this principle. However, periodically poled crystals can also be used in electro-optical applications and acoustic applications. One example of an acoustic optical application is that a Bragg lattice that has a periodically varying diffraction index will occur in a periodically poled crystal in response to a voltage applied across the crystal through the medium of homogenous electrodes. This application can be utilized to modulate or deflect a laser beam, for instance. The poled crystals can also be used as so-called acoustic transducers, i.e. used to generate or to detect an acoustic wave with the aid of the piezoelectric effect. These physical phenomena and applications are well known in themselves, although the periodically poled crystals afford significant advantages and greater possibilities with respect to design of well-functioning components in comparison with standard crystals.
The quasi phase matching, periodically poled crystals relevant in the present context are based on the principle of producing in an existing crystal periodically ordered regions of alternating crystal orientation and therewith periodically varying non-linear optical, electrooptical and acousto-optical properties. This is preferably achieved by applying an electric voltage across the crystal with the aid of a periodic electrode structure, and is thus effected without disintegrating the crystal mechanically.
The published European Patent Application 0 687 941 A2 teaches the formation of inverted or pole reversed ferroelectric domain regions, by applying a ramp voltage across a substrate or crystal, among other things, wherein it has been observed that it is possible to predict an increase in the flow of current through the substrate. Poling can then be considered to have taken place and the voltage is switched-off when the anticipated or predicted current flow has been detected.